Business, science and entertainment applications depend upon computers to process and record data, often with large volumes of the data being stored or transferred to nonvolatile storage media, such as magnetic discs, magnetic tape cartridges, optical disk cartridges, floppy diskettes, or floptical diskettes. Typically, magnetic tape is the most economical means of storing or archiving the data. Storage technology is continually pushed to increase storage capacity and storage reliability. Improvement in data storage densities in magnetic storage media, for example, has resulted from improved medium materials, improved error correction techniques and decreased areal bit sizes. The data capacity of half-inch magnetic tape, for example, is now measured in hundreds of gigabytes on 512 or more data tracks.
The improvement in magnetic medium data storage capacity arises in large part from improvements in the magnetic head assembly used for reading and writing data on the magnetic storage medium. A major improvement in transducer technology arrived with the magnetoresistive (MR) sensor originally developed by the IBM® Corporation. Later sensors using the GMR effect were developed. AMR and GMR sensors transduce magnetic field changes to resistance changes, which are processed to provide digital signals. Data storage density can be increased because AMR and GMR sensors offer signal levels higher than those available from conventional inductive read heads for a given read sensor width and so enable smaller reader widths and thus more tracks per inch. Moreover, the sensor output signal depends only on the instantaneous magnetic field intensity in the storage medium and is independent of the magnetic field time-rate-of-change arising from relative sensor/medium velocity. In operation the magnetic storage medium, such as tape or a magnetic disk surface, is passed over the magnetic read/write (R/W) head assembly for reading data therefrom and writing data thereto.
One problem frequently encountered during reading and writing to tape is that metal particles or fragments of media coatings can come loose from the tape and adhere to the head, leading to formation of metallic bridges on the head. Another problem is formation of metallic bridges via electrostatic and/or electrochemical interaction with the head and involving the tape. Read sensors are particularly susceptible to failure due to shield-shorting as a result of bridging. Conductive bridging appears to occur predominantly in low humidity conditions, e.g., less than about 20-25% relative humidity, depending on the media formulation. Such low humidity conditions are typical with the current prevalence of air conditioned server rooms and business places. It is also believed that tapes having surface resistivities above approximately 106 ohm-cm may aggravate and accelerate the growth of the bridges.
One of the problematic modes is the formation of iron-cobalt bridges that short a lower (S1) shield, which may be nickel iron, Sendust, etc. as is commonly used in tape heads, to the hard bias magnet trim, typically formed of CoPt. The bridges are believed to form from the interaction of Fe and/or Co in the tape with Fe in the shield and Co in the hard bias magnet. Running tape appears to initiate bridge formation. Further, electric fields in the insulation layer between the S1 shield and hard bias magnet film are typically 10-12 volts/micrometer. This large field can drive galvanic corrosion and/or electrostatic deposition.
The only known solutions to the bridging problem are to recess the sensor so that its components do not make electrical contact with the conductive accumulation, coat the head with a durable wear coating, providing wear fences for otherwise non-fenced readers, and/or adjust drive voltages. One apparently reliable means is prerecessing the sensor and applying a coating to the recessed section, but this requires several post wafer machining steps, and so adds cost. Such a method also may produce an undesirable spacing loss. Further, the coating may wear off over time, even with pre-recession.